FXUS66 KSEW 102239
AFDSEW

Area Forecast Discussion
National Weather Service Seattle WA
339 PM PDT Tue Sep 10 2024

.SYNOPSIS...A more fall-like pattern is setting up for the rest of
the week as a series of low pressure systems move through the
area. A passing disturbance brings widespread rain tonight through
Wednesday, with the potential for an isolated thunderstorm as it
moves eastward through Wednesday. Expect cooler temperatures and
abundant cloud cover to continue through much of the week ahead
with occasional shower chances. A stronger front may arrive by
late Monday or Tuesday.

&&

.SHORT TERM /TONIGHT THROUGH FRIDAY/...Clearing skies this
afternoon with temperatures around 70 through much of the interior
at this hour. Meanwhile, increasing clouds are already moving onto
the coast and across the Olympic Peninsula ahead of the next
approaching frontal system. Expect this to bring some leading
light rain to the coast by sunset tonight and into the Puget Sound
region later overnight. Rainfall totals will generally be around a
half inch for the coastal areas and mountains, with closer to a
quarter inch for the remainder of the lowlands from around Seattle
southward. With the cooler air mass moving in, expect that the
upper elevations of Mount Rainier (but not expected down at
Sunrise or lower elevations). Temperatures will be quite cool as a
result of the rain and clouds, with temperatures Wednesday only
topping out in the 60s for many locations. As the low tracks
inland, expect to see steepening lapse rates that could support a
few isolated thunderstorms. Any storms that develop are likely to
pulse up and down quickly with gusty winds and lightning the
primary hazards, and the chances will be highest near the coastal
waters and coastline and south of Puget Sound.

A brief break in the showers and less cloud cover is expected
Thursday as the heights rise a bit in between the weather systems.
This will allow temperatures to nudge a couple of degrees warmer,
or closer to 70 degrees for daytime highs. Not much change to the
overall pattern into Friday.

.LONG TERM /SATURDAY THROUGH TUESDAY/...The region is likely to
remain in a cool and cloudy pattern with a return of precipitation
chances over the weekend and into the start of next week. The
weekend doesn't appear overly wet though, without a strong signal
for a focused round of heavier rain. The next front that
approaches late Monday into Tuesday will then likely bring the
next round of widespread precipitation ot the region with the
ensemble guidance supporting another deeper trough moving across
the Pacific Northwest. Cullen

&&

.AVIATION...Morning fog/cloud coverage has cleared this afternoon
across all terminals (although Whidbey Island and Friday Harbor
continue to see low clouds down to MVFR/IFR). A trough will dig
southward off the Washington coast this evening, and move inland on
Wednesday. This will cause flow aloft to transition from
southwesterly tonight, to northwesterly Wednesday afternoon.

An occluding frontal system will produce widespread rain showers
Tuesday evening through Wednesday, affecting all terminals. In
addition, a few weak thunderstorms are possible over the coastal
waters, and the Southwest Interior/Olympics/South Puget Sound. The
precipitation will move eastward this afternoon over the coastal
waters, reaching the coastline between 23Z-02Z, and Puget Sound as
early as 02Z-04Z. As rain tapers Wednesday morning, CIGs/VIS will
deteriorate. Best chance of IFR/LIFR are areas in/near the
Cascades/Olympics, Strait of Juan de Fuca, and Southwest Interior.
Remaining areas will see low-end MVFR, with overcast skies lifting
to VFR late Wednesday afternoon. Winds will be out of the southwest
through tomorrow morning at 4 to 8 kt, becoming northwest less
than 6 kt late Wednesday afternoon/evening.

KSEA...VFR skies through the afternoon/evening (low clouds continue
to scatter and lift up). Mid/high level clouds will arrive early
this evening, with rain showers arriving as early as 03Z this
evening. Threat for an isolated storm is very low (but possible
heavier precipitation). CIGs/VIS are expected to drop to MVFR
Wednesday morning (lower 1000 ft range), before lifting to VFR
(bases around 3,500 ft) Wednesday afternoon. Southwest winds 5 to 8
kt Tuesday night/Wednesday morning will become northwest less than 6
kt.

HPR

&&

.MARINE...A trough/occluded frontal system will push through the
region Tuesday night/Wednesday. Rain showers have been covering the
coastal waters all afternoon, and will continue to move inland
through tonight/Wednesday morning. A few isolated thunderstorms may
accompany the showers. These may produce gusty winds up to 30 kt
within a 2 to 4 hour window (best chance 9 PM to 2 AM over the
coastal waters - see Marine Weather Statement online). Behind the
rain Wednesday morning, low clouds/fog are likely over the coastal
waters and Strait of Juan de Fuca, through the afternoon.

Once the trough moves through, ridging will rebuild offshore for the
second half of the week with weak onshore/northwest flow over the
waters. The next possible disturbance will move through the region
this weekend, but the threat of any impactful winds is low.

Seas tonight will increase to 3 to 5 feet, and again increase to 5
to 7 feet Wednesday night/Thursday before dropping back to 3 to 5
feet rest of the week.

HPR

&&

.FIRE WEATHER...An upper low pressure system moving in just south of
the area will spread rain showers into this area this evening
through Wednesday evening. Instability associated with the low
pressure trough, and the diffluence aloft, will generate a slight
chance of a few isolated lightning strikes for the southern half of
western Washington. There is a 50 to 75 percent chance of a wetting
rain for the same area.
JBB

&&

.SEW WATCHES/WARNINGS/ADVISORIES...
WA...None.
PZ...None.
&&

$$